26-04-2012, 10:20 AM
DUAL-PORT RAMs SIMPLIFY COMMUNICATION IN COMPUTER SYSTEMS
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INTRODUCTION
Dual-port RAMs allow two independent devices to have
simultaneous read and write access to the same memory.
This allows the two devices to communicate with each other
by passing data through the common memory. These devices
might be a CPU and a disc controller or two CPUs
working on different but related tasks. The dual-port memory
approach is useful and popular because it allows the same
memory to be used for both working storage and communication
by both devices and avoids the need for any special data
communication hardware between the devices. The latest
development in dual-port RAMs has been the appearance of
high speed dual-port RAM chips.
DUAL-PORT RAMS: SIMULTANEOUS ACCESS
A dual-port memory has two sets of address, data and
read/write control signals, each of which access the same set
of memory cells. This is shown in Figure 1. Each set of
memory controls can independently and simultaneously access
any word in the memory including the case where both
sides are accessing the same memory location at the same
time. Up to this time, there have been very few true dual-port
memories available. Memories have a single set of controls
for address, data and read/write logic and are single-port
RAMs. If you wanted a dual-port RAM function, you had to
design special logic to make the single-port RAM simulate a
dual-port RAM in operation.
DMA Limitations: Waiting for the Bus
In a computer system with DMA, the CPU must stop and
wait while an I/O device is doing DMA transfers to memory.
This works well in typical systems where the I/O devices are
transferring data only a small percentage of the time and the
impact on CPU processing time is minimal. These assumptions
do not hold where you have two CPUs trying to use the
same memory. In this case, one CPU must wait while the
other uses the memory. As a result, the average speed of the
CPUs will typically be cut in half.
Dual-Port RAM Chips: How They Work
A true dual-port memory allows independent and simultaneous
access of the same memory cells by both devices. This
means two complete and independent sets of address, data
and read/write logic and memory cells that are capable of
being read and written by two different sources. An example
of the dual-port memory cell is shown in Figure 3. In this cell
both the left and right hand select lines can independently and
simultaneously select the cell for read out. In addition, either
side can write data into the cell independent of the other side.
The only problem would be when both sides try to write into the
same cell at the same time.
Busy Logic Design
Busy logic is called hardware address arbitration logic
because it consists of hardware that decides which side will
receive a busy signal if the addresses are equal. It consists of
Common address detection logic and a cross-coupled arbitration
latch. A logic diagram of the type of busy logic used in the
IDT dual-port RAM chips
SUMMARY AND CONCLUSION
The development of true dual-port memories in integrated
circuit form provides the designer with the ability to set
up communication between components of a computer system
while avoiding many of the problems of prior systems.
While the concept of dual-port memory has been with us from
the early days of computing in the form of DMA, the new dualport
ICs can provide this function at very high speeds and
without the delays associated with earlier designs. Because
of the utility of the dual-port memory concept these chips
should come into wide-spread use and become one of the
standard components used by the computer designer.